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IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 49, NO. 10, OCTOBER 2001 1357
On the Gain of a Reconfigurable-Aperture AntennaElliott R. Brown, Fellow, IEEE
AbstractA full-wave analysis based on the method of mo-ments (MoM) is carried out for a reconfigurable-aperture antennaconsisting of a two-dimensional (2-D) array of filamentarymicrostrip-dipoles interconnected by lossy microelectrome-chanical-system (MEMS) switches. Activation of specific MEMSswitches allows the dipoles to be maintained near the halfwave-res-onant length as the frequency is reduced in octave incrementsbetween 16 and 2 GHz. This keeps the real part of the dipoleself-impedance much higher and the imaginary part much lowerthan in a dipole having a fixed length of at 16 GHz. Hence,the array-antenna gain and aperture efficiency remain muchhigher with frequency than in an array of fixed dipoles. Broadside aperture efficiencies of 3.9, 6.0, 9.5, and 10.6 dB arepredicted for , , , and recap dipole arraysat frequencies of 16, 8, 4, and 2 GHz, respectively, for MEMSswitches having 0.5 dB insertion loss. In contrast, fixed-element
-separated arrays operating at the same frequencies have
predicted efficiencies of 3.9, 24.2, 45.0, and 63.0 dB,respectively.
Index TermsAntenna gain, aperture antennas, reconfigurableantennas.
I. INTRODUCTION
SEVERAL research programs have been started recently to
develop electronic antennas by a new approach called a
reconfigurable aperture, or recap for short. As suggested by
the title, a recap is fundamentally different than the traditional
electronic antennas that have been developed over the years, in-
cluding the family of single-beam phased arrays, multiple-beam
aperture antennas (e.g., Rotman lens), and the growing familyof switchable-element smart antennas [1]. The distinguishing
feature of a recap is its ability to alter the RF current distribution
within a planar-radiating aperture. In the language of phased ar-
rays, a recap can change its element pattern in addition to its
complex-array factor.
To see this distinction more precisely, recall that the electric
(far) field from a traditional-phased array can be written [ 2]
(1)
where
;
distance (radial unit vector) between the center
of the array and the measurement point;
Manuscript received August 5, 2000; revised November14, 2000. This workwas supported by the Defense Advanced Research Projects Agency under theReconfigurable Aperture Program.
The author is with the University of California, Los Angeles, Los Angeles,CA 90095-1594 USA (e-mail: [email protected]).
Publisher Item Identifier S 0018-926X(01)06368-2.
vector between the th element and the measure-
ment point;permeability of free space;
propagation constant ;
element factor;
array factor;
total number of elements in the array.
In traditional phased-array antennas, the element factor is fixed
by design and cannot be altered electronically or otherwise. The
far-field pattern is changed only by variation of the complex
coefficients in phase, amplitude, or both. In a recap one can
also change the element factor.
Optoelectronic [3] and MEMS switches [4] have already been
used to change the length of single-element antennas, such as
dipoles. Similar devices have been proposed to change the el-ement factor in arrays [5], but it is difficult to tell at this point
which will work best. However, two benefits of a recap, inde-
pendent of the core technology, should be the antenna gain as a
function of beam pointing and as a function of frequency shift.
The pointing issue has already been addressed through the de-
velopment of a broad-side/end-fire switchable antenna based on
an integrated leaky-mode/YagiUda structure [6] and willnotbe
addressed here. Instead, this paper focuses on the issue of fre-
quency shift and how a recap array of resonant elements can
maintain high gain over a wide (reconfiguration) bandwidth by
varying the element length and interelement separation to stay
at or near resonance at each frequency. This does not imply an
increase in the conventional gain-bandwidth product in whichthe bandwidth must be instantaneous. But for some applica-
tions this distinction may not be so significant if the reconfigura-
tion time is sufficiently small. For example, with contemporary
MEMS switches this time would be s, which is adequate
for many communications systems.
The pervasiveness of military and commercial satellite com-
munications in Ku band (1218 GHz) and the explosive growth
of personal communications services (PCS) just above 2 GHz
define an interesting application of a reconfigurable aperture as
an electronically-steerable antenna that can link to space-based
or terrestrial transceivers. This application will define the spe-
cific frequency range and other parameters in the simulation de-
scribed below.
II. CANDIDATE ARCHITECTURE
The primary purpose of this paper is to analyze the gain and
impedance characteristics of a recap antenna in comparison to
the diffraction limit and to a fixed-element antenna having iden-
tical architecture and materials properties, but lacking the recon-
figurability. To facilitate the analysis, the simple recap architec-
ture shown in Fig. 1(a) was chosen. It consists of a square lat-
tice of rectangular microstrip elements, each having
0018926X/01$10.00 2001 IEEE
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1358 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 49, NO. 10, OCTOBER 2001
(a) (b)
Fig. 1. Recap architecture consisting of microstrip elements that can be connected by MEMS switches or to a balanced transmission line to form an array ofplanar-dipole antennas. (a) Maximum-frequency ( ) configuration where the gaps between microstrip elements are alternately opened and connected to abalanced line to form an array of approximately -long, -spaced dipoles. (b) First subharmonic configuration where 50% of the gaps are closed (by aMEMS switch), 25% are opened, and 25% connected to a balanced line to form an array -long, -spaced dipoles at .
length along the -axis and filamentary width (i.e., )
along the -axis, and each separated from its neighbors along
the -axis by an infinitesimal gap (i.e., ). Located at
each gap is a series-connected MEMS switch and a shunt con-
nected balanced transmission line. The figure shows a top view
of this array in the maximum-frequency configuration
in which all the MEMS switches are left open and every other
gap is coupled to RF so that the radiating aperture consists of
a square lattice of microstrip dipoles having length .
The unit cell of the square lattice is defined by the dashed box
(width ). In principle, this array can be quite efficient and
electronically steerable over wide angles under the conditionof half-wave resonance, , where is the
wavelength in free space, and is the
permittivity of the dielectric substrate material (assumed loss-
less). Hence, the maximum frequency configuration is defined
by .
Fig. 1(b) shows the array in the first subharmonic
configuration in which half of the MEMS switches are closed
and half of the remaining gaps are driven with balanced RF
so that the radiating aperture consists of -long microstrip
dipoles lying on a square lattice (unit cell width ). Be-
cause the length of the dipoles has approximately doubled com-
pared to those in Fig. 1(a), this configuration should be reso-
nant at a frequency . Furthermore,if the number of microstrip elements is large, lower subhar-
monic configurations can be produced by judiciously switching
some gaps and coupling RF to others. In each case, the recap
array is reconfigured as a square lattice of half-wave dipoles
with approximately half-wave center-to-center element sepa-
ration. The lowest subharmonic frequency that yields an elec-
tronically-steerable array, and the one called the minimum-fre-
quency configuration, is the array defined by
.
As the recap is configured for subharmonic frequencies, the
number of activated switches must increase in a manner
described analytically by the expression .
TABLE IPROPERTIES OF THE AND THREE SUBHARMONIC CONFIGURATIONS OF
RECAP ARRAY of 32 16 MICROSTRIP ELEMENTS ON A SUBSTRATEHAVING AND THICKNESS mm
For example, , 6, 14, and 30 for the 1st, 2nd, 3rd, and
4th subharmonics, respectively. The switches have a small butsignificant value of insertion and return loss that is ultimately
an important factor in the useful bandwidth of a recap antenna.
III. SIMULATION PARAMETERS AND METHODOLOGY
As suggested above, an interesting application of a recap is
an electronically-steerable antenna that can link to terrestrial or
space-based transceivers over a range between an of 16
GHz and an (3rd-subharmonic) configuration of 2 GHz.
From the relations given above, an array of microstrip
elements will cover this range by providing a
dipole array at 16 GHz and a dipole array at 2 GHz.
At the intermediate subharmonic frequencies of 8 and 4 GHz,and arrays will be available that also satisfy the
-length, -spacing of the dipoles. The characteristics
of the maximum frequency and three subharmonic configura-
tions are listed in Table I, along with the number of switches
per dipole required in each configuration.
The next parameters chosen were the permittivity and
thickness of the substrate material. Through extensive research
conducted in the 1980s, it was shown that high- substrates are
usually deleterious to the performance of microstrip antennas
(dipoles and patches in particular), because of their propensity
for surface modes [7], [8]. It was also shown that there is
always at least one surface mode present (the TM ) and that
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BROWN: ON THE GAIN OF A RECONFIGURABLE-APERTURE ANTENNA 1361
TABLE IIISELF-IMPEDANCE AND NEAREST NEIGHBOR (COLLINEAR) MUTUAL IMPEDANCE VALUES OF PLANAR MICROSTRIP DIPOLES IN FIXED AND RECAP ARRAYS
effect of the ground plane, which at 8 GHz is now 50% closer
to the antenna (relative to ) than it is at 16 GHz. These
trends continue at the lower subharmonics, culminating in a
fixed element, self-impedance at 2 GHz of 0.01212180 and
a recap self-impedance of 1.352.69 .
After obtaining the elements, the current components
were computed by matrix inversion of (2) using standard Matlab
routines. Then the antenna gain was computed from (11) using
(8)(10) as input. Plotted in Fig. 2 is the resulting gain for
(16 GHz) and each subharmonic configuration in comparison
to the diffraction-limited gain at 16, 8, 4, and 2 GHz and to
the fixed-array (dipole length cm) gain at the same fre-
quencies. The corresponding gain values are listed in Table IV.
The recap gain is parameterized by the MEMS switch insertion
loss, ranging between 0.0 and 3.0 dB. For zero MEMS loss, the
gain of the recap array falls at approximately the same rate with
frequency as the diffraction limit, namely . Hence, the
aperture efficiency , remains around dB. At
first this appears surprising in light of the fact that the real part
of the recap self-impedance drops steadily from 61.6 to about
1 between 16 and 2 GHz. However, while the real part drops,
the mutual impedance elements remain relatively unchanged, as
displayed through the nearest-neighbor mutual-impedance term
in Table III. Hence, the mutual coupling between adjacent ele-ments increases, and an increasing fraction of the input power
to a given element is radiated by its neighbors.
In contrast to the recap behavior, decreasing the fre-
quency in a fixed-element array decreases the real part of the
self-impedance while increasing the (capacitive) imaginary
part at a comparable rate. This is evident in Table III where
at all subharmonic frequencies the self-impedance term is
dominated by a large capacitive reactance. Not only does this
present a larger return loss to a 50- generator than a recap
element, but it is much less favorable for mutual coupling. This
point is also demonstrated in Table III where one sees that the
self-impedance terms for the fixed-element array dominate
the mutual impedance terms (collinear nearest neighbors) atall frequencies, making mutual coupling rather ineffective in
transferring power from one element to its neighbors.
Another interesting aspect of Fig. 2 pertains to the effect of
MEMS switch loss on the gain and aperture efficiency. It is
remarkable that no level of switch loss simulated here can re-
duce the recap gain to the fixed-element gain. Thus, an inter-
esting figure of merit is the level of switch loss at which the
recap gain and the fixedaperture gain become equal. Using
the definitions given above, the switch loss will be given by
the expression , where
is the gain of the recap configuration with zero switch loss.
Solving for the insertion loss per switch, one finds
Fig. 2. Antenna gain as a function of frequency for the recap array, afixed-element array, and a diffraction-limited aperture ( ) with a
cm . The gain for the recap and fixed-element arrays are computedonly at the maximum frequency (16 GHz) and the first three subharmonics (8,4, and 2 GHz). The lines connecting the data points are drawn only as a visualaid. The recap gain is parameterized by the MEMS switch insertion loss ( )that ranges between 0 and 3.0 dB. The MEMS return loss is assumed to benegligible.
. The resulting values for are 9.6, 6.4,
and 4.2 dB at 8, 4, and 2, GHz, respectively.
V. SUMMARY
This paper has analyzed two-dim arrays of planar microstrip
dipoles as the frequency is varied in octave increments between
16 and 2 GHz. Two different array structures were considered:
1) a fixed-element aperture in which the dipole length is con-
stant at the maximum frequency (16 GHz)
and 2) a recap in which the dipole length is maintained near
over several octaves of bandwidth by judicious activation
of MEMS switches between the elements. In both structures, the
inter-element separation is maintained at . For the fixed-el-
ement aperture, the gain and aperture efficiency are found to de-crease rapidly with frequency because of a rapid increase in the
return loss arising from impedance mismatch between the gen-
erator(s) and dipole elements. The impedance mismatch is as-
sociated with a rapid drop in the real part of the self-impedance
and a large (capacitive) increase in the imaginary part. And be-
cause the mutual impedance elements all remain relatively
small, there is little radiation into free space by either a driven
element or its neighbors.
In contrast, for low switch insertion loss, the recap array
maintains a high gain with reduced frequency that nearly tracks
the diffraction limit ( ). This is because the switches
maintain the length near the resonance, which keeps the
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1362 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 49, NO. 10, OCTOBER 2001
TABLE IVANTENNA GAIN FOR THE FIXED ELEMENT AND RECAP ARRAYS VERSUS FREQUENCY AND PARAMETERIZED BY MEMS SWITCH INSERTION LOSS ( )
real part of the self-impedance relatively high and the imagi-
nary part relatively low. In addition, the mutual impedance to
neighboring elements, particularly the collinear term is
relatively large and approximately constant with decreasing
frequency. Hence, the recap maintains high efficiency with
reduced frequency largely because each dipole maintains an
acceptable return loss and couples increasingly to its neighbors
in a constructive way, at least for the broadside radiation pat-
tern. Future research will analyze these effects in the presence
of electronic beam steering.
In conclusion, it should be noted that the critical choice ofsubstrate permittivity ( ) in this analysis was driven
by performance and economic considerations. Higher- mate-
rials, such as high-resistivity Si or SIGaAs, would likely yield
inferior performance of the recap antenna compared to that of
Fig. 2 because of deleterious surface-wave effects. However,
MEMS switches are now being developed primarily on such
high- materials. Hence, application of the low- substrate
simulated here would require that semiconductor-based MEMS
be bonded by flip-chip or similar packaging technology. This
approach may be feasible over the simulated frequency range
where the size of the MEMS die should be a small fraction of a
wavelength and, therefore, have little effect on the behavior of
the planar antennas.
ACKNOWLEDGMENT
The author would like to thank T. Itoh and Y. Rahmat-Samii
of UCLA for helpful discussions on this subject.
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Elliott R. Brown (M92SM97F00) received theM.S. and Ph.D. degrees in applied physics from Cali-fornia Institute of Technology, Pasadena, in 1985 and1981, respectively.
He is a Professor of Electrical Engineering at theUniversity of California, Los Angeles (UCLA) and iscurrently conducting research projects in RF powerelectronics and thermal management, RF recon-figurable antennas, MEMS ultrasonic transducersfor biomedical imaging, shot noise suppression
in semiconductor devices, and THz electronics and optoelectronics. Beforejoining UCLA, he was a Program Manager at DARPA in Arlington, VA. Prior
to DARPA, he was with Massachusets Institute of Technology (MIT), LincolnLaboratory, Lexington, MA, where he conducted and managed research insolid-state science and technology.
Dr. Brown is a member of the American Physical Society.